The present invention relates to ring lasers and, more particular, output beam combining apparatus for forming a single beam from the two or more output beams of a ring laser.
Ring lasers have become widely used in a variety of applications, often related to the measurement of rotational motion and acceleration. The ring laser typically forms a closed polygonal optical path within which two laser beams propagate in opposing directions. This optical path may lie in a single plane (i.e., flat) but need not. Rotation of the ring laser causes a frequency shift in each of the counter propagating laser beams. The magnitude and direction of this rotation can be determined by combining the counter-propagating laser beams outside of the ring laser optical path and measuring a resulting beat frequency.
Ring lasers are usually made from a solid block of material having a series of intersecting bores to form the optical path. Several mirrors are disposed at the intersections of these bores to direct the laser beams about the optical path. Typically one of these mirrors allows a small amount of each laser beam to escape the ring laser so that the rotationally-induced beat frequency can be measured. This output mirror is usually formed by evaporating a partially reflective coating onto an interior surface of an optically transmissive substrate. Consequently, as the counter-propagating laser beams exit an exterior or rearward surface of the output mirror, the two beams are physically displaced and diverging. Output beam combining apparatus are normally used outside the ring laser structure to re-combine these diverging laser beams into a single beam and usually also redirects this beam to a suitable detector.
Conventional output beam combining apparatus typically include a retro-reflective device. Retro-reflectors possess the characteristic of reflecting a light beam parallel to, but displaced from, an incoming or incident light beam. The retro-reflector is oriented so as to direct one of the exiting laser beams back toward the output mirror to a point on the exterior surface of the mirror where the counter-propagating laser beam emerges so that a portion of the first laser beam, returned by the retro-reflector, is combined with the second laser beam, exiting the laser, to form a single beam. This single beam is then directed to a detector that measures the beat frequency of the combined beams during rotation.
Although suitable for combing the separate and diverging output beams of the ring laser, output beam combining apparatus employing retro-reflectors have their drawbacks. Typically the retro-reflector structure includes three polished surfaces oriented at right angles to one another within tolerances of a few arc seconds. Fabrication of this relatively complex structure is normally expensive, with the cost increasing as the size of the retro-reflector decreases.
Thus there exists a need for a less complex and more inexpensive output coupling apparatus for a ring laser. The present invention provides this device.